When a force does work
You already know this much: if a force acting on an object changes where the object is, or changes its shape, then that force has done work. This chapter takes the idea further and puts a number on it.
The two signs that work was done
Apply a force to something and watch what happens to it. Either of these two changes means work was done.
Its position changed
The object is somewhere it was not before — pushed along a table, lifted off the ground, dragged across a floor.
Its shape changed
The object is the same place but a different shape — a stretched rubber band, a bent metal strip, a squashed ball of clay.
A force on its own is not enough
Notice that the force alone does not settle it. Something has to have happened to the object as a result. That is why the quantity you are about to meet multiplies the force by the distance it moved through, rather than just recording the force.
Pushing a wall is not work
Push as hard as you like against a wall that does not move: in the physics sense you have done no work at all, because nothing moved and nothing changed shape. You will feel tired, which is a different matter entirely — your muscles are doing work inside your own body.
Work needs a force and a change
A force that produces neither movement nor a change of shape does no work, however large the force is.